System and method for optimized channel switching in digital television broadcasting

ABSTRACT

A system and method for masking/reducing the switch time from an analog or digital television channel to another digital television channel and back. Various optimizations which may be used individually or in combination to minimize the potential disruption to the viewer. This optimization is especially important when the channel switch is made automatically by the receiver, such as the case where the presentation of one television program is temporarily continued on another channel.

RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 10/423280, filed Apr. 25, 2003, which claims the benefit of U.S.Provisional Appln. No. 60/375562, filed Apr. 25, 2002, each of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention is directed towards digital television, and moreparticularly towards a method for switching between two televisionchannels where at least one of the channels is digital.

BACKGROUND

Channel switching between television channels as being received by atelevision receiver (such as a television, a set-top box, . . . )usually takes a certain amount of time, typically between 1.5 and 2seconds, depending on the signal strength and receptionhardware/software. During this switch time the video is usually black(blanked) and the audio is silent.

In normal situations, where the viewer triggers the channel switch(e.g., by pressing channel up/down buttons on the remote control), thisperiod of silence/blackness is no problem since it is expected by theviewer, and also because the television programs that are being switchedbetween have no relation between each other (are independent). In such asituation it is still desirable, however, to reduce the switch time(which is defined here as the time between presentation of the lastvideo/audio from one (source) channel and the presentation of the firstvideo/audio front the channel that is switched to).

In situations where the channel switch is made autonomously by thetelevision receiver, for example because the presentation of one programis temporarily continued on another channel it is more important thatthe switch time is as short as possible, since the switch disrupts theflow of the presentation and the viewer's experience since (s)he doesnot expect a switch to happen. An example of such a situation isdescribed in the Visible World patent application “System and Method forSimultaneous Broadcast for Personalized Messages”, filed on Apr. 24,2001 with application Ser. No. 09/841,465 and is incorporated hereto byreference, which describes how to implement personalized messaging in abroadcast environment in a highly bandwidth efficient manner by usingmodular (slot-based) messages. In one embodiment, personalized messagesare concentrated in one (or a few) dedicated digital streams, so thatexisting analog and digital television channels can “share” personalizedmessages by switching to and from these dedicated channels at theappropriate times. This leads to a highly bandwidth efficient systemsince the personalized messages are time-shared on one (or a few)channels.

The present invention describes methods for reducing the channel switchtime from an analog or digital channel to a digital channel, and back.

Note that the related Visible World patent application “System andMethod for Seamless Switching”, filed on Dec. 13, 2000 with applicationSer. No. 09/735,983 and incorporated herein by reference, explains howto seamlessly switch between content elements (modules of personalizedmessages) within a digital stream. The present application disclosesmethods on how to seamless switch to such a digital stream from anotheranalog or digital stream and back.

SUMMARY

The present invention addresses issues around masking the switching froman analog or digital television channel (located at a certain frequencyin the frequency spectrum) to another, digital, television channel(located at a different frequency in the frequency spectrum) and back.In normal television broadcasting, channel ‘zapping’ between twounrelated channels can take up to 2 seconds in time, while the TV set(or set top box, STB) is performing the channel switch.

The present invention includes general methods to reduce such switchtime, as well as specific methods that can be employed in situationswhere there is a relation between the channels that are being switchedbetween.

The general methods involve using audio/video data in digitalaudio/video decoder buffers at the moment the switch is being made tocover up at least the tuning and demodulating parts of the switch time.

The specific methods involve the preparation/conditioning of the digitaltarget stream (that is being switched to) such that playout of itsaudio/video can start faster than in the general case, by exploiting howvideo is encoded.

An illustrative embodiment of the present invention is described thatinserts a personalized message, such as a commercial, in a televisionprogram in a bandwidth efficient manner.

An illustrative embodiment of the present invention for a broadcasttelevision receiver including a tuner for selecting one channel frommultiple channels and an encoded media buffer for receiving digitalencoded media from a channel selected by the tuner, includes a method ofswitching from a first digital channel to a second channel. The methodincludes inputting digital encoded media from the fast digital channelselected by the tuner to the encoded media buffer, then halting input ofdigital encoded media from the first digital channel to the encodedmedia buffer, while continuing to output digital encoded media from thefirst digital channel from the encoded media buffer. This also includesswitching the tuner to the second channel, and after a passage of timefor the tuner to complete switching to the second channel, outputtingthe second channel from the tuner. If the second channel is a digitalchannel, then outputting the second channel from the tuner includescommencing input of digital encoded media from the second channel to theencoded media buffer. If the second channel is an analog channel, thenoutputting the second channel from the tuner includes bypassing theencoded media buffer.

The present invention also includes situations wherein the passage oftime for the tuner to complete switching to the second channel is apredetermined amount of time, and a quantity of digital encoded media inthe encoded media butter is maximized to cover a maximal amount of thepredetermined amount of time. This minimizes the amount of time theviewer sees no media. The present invention also includes attenuatingaudio for the first channel before switching the tuner to the secondchannel, increasing audio volume for the second channel afterwards.

When the media is digital video, and the second digital channel is anMPEG encoded channel, an illustrative embodiment includes creating thesecond digital channel such that after switching the timer to the secondchannel, upon commencing input of digital encoded video from the seconddigital channel to the encoded video buffer, the first input into theencoded video buffer is a complete MPEG closed Group of Pictures (GOP).This avoids the system needing to wait for a GOP (I-frame).Alternatively, digital video can be previously encoded such that aVBV-delay of a first video frame in presentation order is reduced. Thismay be accomplished by increasing the video bitrate, and/or reducing aVBV buffer size maximum value for an encoder.

Another illustrative embodiment includes sending at least one controlmessage for the second channel to the broadcast television receiver at atime before switching (or completion of switching the tuner to thesecond channel. This avoids the system needing to wait for controlmessages to appear in the channel that has been switched to.

The present invention works whether the media is video, MPEG encodedvideo, audio, MPEG encoded audio, or AC-3 encoded audio. An illustrativeembodiment of a broadcast television receiver (included full televisionset, set top box etc.) includes a tuner, to select one channel frommultiple channels received by the receiver. It also includes a digitalencoded media buffer coupled to the tuner; the digital encoded mediabuffer to receive digital encoded media when a first digital channel isselected by the tuner. Other components may be positioned between thetuner and the digital encoded media buffer, such as switches, digitaldemodulator, demux/descrambler etc. When the tuner is switching to asecond channel, the digital encoded media buffer ceases to receivedigital encoded media for the first digital channel, but continues tooutput digital encoded media for the first digital channel through anoutput. After the tuner has completed switching to the second channel,if the second channel is a digital channel then the digital encodedbuffer receives digital encoded media for the second channel. If thesecond channel is an analog channel, then output from the tuner bypassesthe digital encoded media buffer.

An advantage of the present invention is the ability to provide atelevision viewer with an uninterrupted presentation, without longperiods of silence/blackness while the channel switches are beingperformed by the receiver. The television viewer is minimally impactedduring such changes, and may not even notice. Even if such channelswitches are not completely masked, the present invention allowsminimization of the effect of such channel changes.

An additional advantage of the present invention is the ability todistribute portions of a television program between different channelsto achieve bandwidth-efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the present inventionwill be more fully understood from the following detailed description ofillustrative embodiments, taken in conjunction with the accompanyingdrawings in which:

FIG. 1 illustrates an example of a television channel lineup (frequencymap).

FIG. 2 illustrates the logical components on the digital signal path ina digital television receiver.

FIG. 3 a illustrates the logical components on the analog signal path ina television receiver.

FIG. 3 b illustrates the logical components of signal paths in adigital/analog television receiver.

FIG. 4 illustrates the buffer fullness graph of an MPEG video decoderbuffer at the beginning of playback (e.g., after a channel switch iscomplete).

FIG. 5 illustrates the buffer fullness graph of an MPEG video decoderbuffer at the end of playback (e.g., when a channel change isrequested).

FIG. 6 illustrates the addition of an additional tuner in the televisionsignal path to reduce the period of silence/blackness during a switch.

FIG. 7 illustrates audio ramp up/down when making a channel switch.

FIG. 8 illustrates a broadcast television headend system that allowsdigital receivers to replace a default message in a current televisionchannel by an alternative message on another channel.

FIG. 9 illustrates duplication of access control messages.

FIG. 10 illustrates channel switching process in a digital receiver, and

FIG. 11 illustrates channel switching process in a digital/analogreceiver.

DETAILED DESCRIPTION

The present invention finds utility in various data transmissionapplications including, but not limited to, transmission, reception anddecoding of broadcast television signals (whether distributed via cable,terrestrial, satellite, microwave, etc.); encoding, multiplexing, anddecoding of MPEG (Moving Pictures Expert Group) and AC-3 (Audio Codingalgorithm, third generation) based multimedia streams. The invention isboth applicable in distribution of television signals to the consumerhomes as well as distribution from a originator to affiliate stations(such as cable headends).

Frequency Maps

Television programming (as well as other content) distributed via atelevision distribution system, is carried to the consumer's homes via aplurality of frequency-multiplexed channels. Figure 1 shows a typicalexample of the subdivision of the frequency spectrum in a cabletelevision system. The frequency spectrum in this example contains:

A plurality of analog television channels 102. Each channel conveysexactly one single analog television program (or pay-per-view movie,etc.)A plurality of digital television channels 103. Each channel conveys aplurality of televisions programs, depending on the amount ofcompression used (typical is 6-10 programs in one channel). Suchprograms can also be pay-per-view movies, VOD (video On Demand) movies,etc. Digital channels contain television programs that are digitized,compressed, and multiplexed (using techniques such as MPEG), and theresulting bit stream is then mapped on a certain frequency band(channel) for distribution using modulation techniques such as QAM(Quadrature Amplitude Modulation), QPSK (Quadrature Phase Shift Keying),8-VSB (8-level Vestigial Side Band), COFDM (Coded Orthogonal FrequencyDivision Multiplexing), etc.

A plurality of digital data channels 104. Data channels are assigned tocarry data traffic such as Internet data (e.g., for DOCSIS modems),telephone traffic, etc.

Forward Out-Of-Band portions 101 assigned to forward traffic, e.g., usedto control television receivers (send time updates, send conditionalaccess control messages, IP traffic. receiver specific messages, etc.)Such traffic is typically modulated differently than the digitaltelevision channels. This data is also known as ‘Out-Of-Band’ traffic.

Reverse Out-Of-Band portions 100 assigned to return traffic (e.g., VODcontrol commands, telephone data, IP data, etc.).

Note that the example is specific to a US Cable network, yet a similarpicture could easily be derived for other networks such as SatelliteDTH, Terrestrial, Non-US Cable, Distribution Networks for Broadcasters(e.g., to affiliate stations), etc. Some television broadcast systemshave return paths, others have not. Some have analog channels, some areonly digital.

Common for a single distribution systems is that all (non-OOB) forwardchannels are carried in frequency bands of a fixed size (6 MHz in FIG. 1is typical for the USA, while other sizes, such as 8 MHz can be seen inEurope).

In a system utilizing the present invention at least one of the channelswill be digital, and the receiver unit must be capable of receiving anddecoding digital television signals. Such a receiver is typically adigital consumer set-top hex (STB) or a digital television set.Alternatively, it can be a commercial receiver as placed in headends oftelevision broadcast networks. Such commercial decoders receive digitalsignals from their point of origination and manipulate these signals forfurther distribution down the network.

Digital Television Receivers

The digital signal processing path in a digital receiver is illustratedin FIG. 2.

Tuner 200 is responsible for selecting the frequency band at which thedesired program is carried. For Cable Receivers this may be a QAM tuner,for Satellite a QPSK tunes, etc. The tuner produces an analog signal(sometimes called Intermediate Frequency, or IF) that goes info thedemodulator 201.Demodulator 201 is responsible for demodulating the incoming signalbased on the specific modulation scheme used (QAM-64, QAM-256, QPSK,etc.). The demodulator extracts (demodulates) the digital signal fromthe analog signal, and passes that digital signal (typically an MPEG-2transport stream or a variant thereof) on to the Demux/Descrambler 202.Demultiplexer/Descrambler 202 is responsible for selecting the desiredaudio, video, and data from the incoming bitstream, and (if scrambled)de-scrambling it. The Demux/Descrambler produces unscrambled, encoded,digital audio and video streams. It also extracts data-messages 211 fromthe incoming stream. Audio is typically encoded in AC-3 or MPEG formats.Video is typically encoded in MPEG format. The streams as output by theDemux/Descrambler go into a video buffer 203 and an audio buffer 207.Video decoder 204 reads from the encoded video buffer, decodes thevideo, and writes the resulting uncompressed video to a buffer 205. Theaudio decoder 208 does the same for audio, and puts it in a buffer 209.Video renderer 206 reads from the uncompressed video buffer, and rendersthe video in a timely fashion (one frame every frame time, e.g. 29.97times per second in NTSC (National TV Standards Committee)). The outputof the renderer typically is an (analog) television video signal thatcan be displayed on a television screen. The audio renderer 210 in asimilar fashion reads uncompressed digital audio from the buffer 209 andgenerates a conventional analog audio signal that can be input tostandard consumer audio equipment such as an audio amplifier.

It should be noted that this diagram identifies the logical componentsthat participate in the basic digital signal flow. In realimplementations some of these functions may be combined (e.g. tuner anddemodulator into one chip) and/or implemented in hardware or software(for example, the video decoder can be a hardware device or a softwareprogram, or combination thereof).

Most digital television receivers also support the reception of analogchannels. This is specifically true in situations where analog anddigital channels co-exist (for instance in a cable network where analogand digital programs are carried on the same cable infrastructure (asdepicted in FIG. 1), or in direct-to-home, where the digital channelsare carried on satellite and the analog channels are carried terrestrialin an entirely different part of the frequency domain.

The signal path for reception of analog channels only is shown in FIG.3. Depending on the modulation scheme used, the tuner 200 can be sharedwith the digital signal path (the tuner output is switchable to theanalog or digital signal path), or it can be separate (such as in thedirect-to-home case).

Tuner 200 is responsible for tuning to the frequency band that theprogram is carried at. The tuner produces analog signals (video, audio,and data such as Closed Caption in the USA or Teletext in Europe carriedin the VBI interval).Video processor 301 receives the analog video signal and converts itinto a displayable format. This might include descrambling, colordecoding, 50-100 Hz conversions, etc. Similarly, audio processor 302receives the analog audio signal and converts it into a standard formatthat can be put onto speakers or can be fed into audio equipment.

Note that the processors 301 and 302 might be split-up into multiplesmaller physical components. Alternatively they might be integrated intoone big unit (e.g., one single chip).

FIG. 3 b shows the combination of analog and digital signal paths in ananalog/digital television receiver. Also shows in this Figure areswitches 304 and 305 in the backend, which are used to select whichsignals (either from the digital path or from the analog path) tooutput.

Channel Switching

Here and in the remainder of this disclosure, channel switch time isdefined as the time between presentation of the last video frame/audiosample front one (source) channel and the presentation of the firstvideo frame/audio sample from the channel that is switched to.

In the generic case, when switching from any (analog or digital) channelto an analog channel, the following steps take place in a televisionreceiver:

1. Before actually timing, the receiver switches off the video and audioprocessors 301 and 302 and thus turns the output signals toblack/silence.2. The tuner selects (re-tunes to) the new frequency.3. The processors start processing and rendering the audio and video(via processors 301 and 302) as soon as the output signal of the tuneris stable.

The typical amount of time necessary for changing to an analog channelcan range between 200 and 1000 msec (or more), mainly depending on thequality of the signal and the tuner hardware/software.

Switching from any (analog or digital) channel to a digital channelinvolves considerably more steps in the general case:

1. Before actually tuning, the receiver switches off (and resets) thevideo 204 and audio decoders 208 and thus turns the output signals toblack/silence.2. The tuner 200 selects (re-tunes to) the new frequency and thedemodulator 201 synchronizes on the incoming stream (in the Cable casethis is called a “QAM lock”). This step can take anything between 200msec up to 1000 msec (or more), depending on the tuner/demodulatorhardware, the software controlling the tuner/demodulator, and thequality of the incoming signal.3. The Demux/Descrambler 202 extracts a number of basic control messagesfrom the stream. These control messages describe the contents of thedigital stream in terms of program numbers, location of the programaudio/video, etc. The messages enable the receiver to “find” therequested television program in the multiplex. In MPEG-2 systems thesewould for instance be the PAT (Program Association Table) and PMT(Program Map Table) messages. The typical time needed to find suchmessage is on average around 20 msec, but can be as much as 50 msecsince these messages appear in the bitstream at fixed time intervals.4. Next, the decoders 204, 208 synchronize on the incoming video/audiostreams. For instance in an MPEG-2 system the video decoder 204 has towait for the next I-frame before it can start filling the encoded databuffer 203. All data before that first I-frame must be discarded. Sincethe interval between two subsequent I-frames can be as much as 18 frames(or more) in typical encoded MPEG video, the average wait time can be asmuch as 9 frames (300 msec). For the worst case, the wait time can bemore than half a second.5. The buffers 203 and 207 holding encoded (compressed) data will haveto be filled to a certain extent before decoding can start, in order toavoid buffer underflows. For instance in an MPEG-2 system, the videobuffer 203 will have to be filled according to the initial VBV delay ofthe first frame. This can be as much as 300-400 msec in conventionallyencoded MPEG video, depending on the bitrate chosen. The VBV bufferfillrate 404 at the start of a new stream is illustrated in FIG. 4. ThisFigure shows a VBV buffer graph for a video decoder with buffer size 401(e.g., 224 Kbyte). At time 402 new valid data starts entering thebuffer. At time 405 a the first frame of encoded data is taken out ofthe buffer by the video decoder. In total, it takes a time 403 untilthis first frame is taken out of the buffer.6. The decoders 204, 208 can now start decoding video and audio, andnext the renderers 206, 210 can start rendering video and audio. In casethe first frame in decoding order is not the first one in presentationorder an additional frame time must pass before actual frames areavailable for display. In FIG. 4, for example, it could be that the Pframe at time 405 b is the first frame to be presented. The I frame thatis taken out of the buffer at time 405 a (i.e., the first in decodingorder) appears later in the presentation order.

As can be seen, the switch time to a digital channel can take aconsiderable amount of time, and can be as mach as 2 seconds or more(especially if SW control overhead is included). Switching to an analogchannel typically takes less time since no extensive digital processingsteps have to be performed, but can still be up to 1.5 seconds of timeas explained above.

Such long delays between changing channels (and having no video/audiopresent) is disruptive to the experience of the viewer and should beminimized. This holds for the general case of channel zapping, but evenmore in cases where the programming on the different channels isrelated.

The present invention discloses methods that can be used to reduceand/or mask the time needed for the switch. The invention focuses onmethods that can be applied to today's digital television receivers(set-top boxes, commercial receivers, or other) without hardwaremodifications.

It should be noted that the methods described in this document work forboth directed channel change (e.g. channel change initiated by thesystem, rather than the local viewer), and for more standard viewerinitiated channel change (zapping).

The methods will work for all situations where a digital receiver isdeployed, be it a receiver in the home (e.g., set top boxes, digitaltelevision sets, etc.) or a receiver in the distribution network (e.g.commercial receiver at cable plant, commercial receiver at localbroadcasting station, etc.).

In the general case (basically the normal situation of a viewer, or thebox autonomously. zapping between two channel) there are a fewopportunities for masking/reducing the switch time as perceived by theviewer.

Method 1 (General): Dual Tuner Receivers

The first method to reduce the switch time works in receivers with twotuners as depicted in FIG. 6. Using this method, the second tuner tonesto the new desired channel (frequency), while the first tuner stays onthe original frequency. Only after the second tuner is tuned and has astable signal, the system switches to the output of the first tuner,using a programmable switch 603. This solution will provide continuousplayback in the system while the second tuner is tuning, thus reducingthe amount of silence/blackness observed by the viewer. This solutionworks regardless from the type of switch (analog or digital to analog ordigital).

When the switch is from an analog to a digital channel (or vice-versa)and the system has separate tuners for analog and digital channels, thesignal paths are essentially separate, and only at the backend of thesystem it is decided which signal to route to the outputs of thereceiver via switches 304 and 305. In this situation the solution toreducing the amount of silence/black when switching from analog todigital or from digital to analog is making the switch in the backendonly AFTER the new signal path is producing audio/video, thusessentially reducing the switch time to zero.

Method 2 (General): Using Digital Video Buffer

In a situation where the source channel is digital, and the targetchannel analog or digital, the buffers in the digital signal path can beused to reduce the perceived switch time. As illustrated by FIGS. 2 and4, the buffers 203 and 207 will have some audio/video buffered. In thesituation of video, buffer 203 can have as much as 200-400 msec, worthof video left before the buffer is empty. This data can be played whilethe tuner 200 is tuning, thus to cover up the tune itself. This wouldmean that the output of the tuner is temporarily switched off (orignored), while the video and audio decoders keep playing the contentsof their respective buffers. In FIG. 5 this means that a switch at time502 will lead to a time 504 of presentation until the screen goes black.This is further illustrated in FIG. 5. The total time that the data canstill be played is the so-called ‘ending delay’, which is 503 in FIG. 5.

As shown in FIG. 10, the normal procedure taken by receiver controlsoftware 1003 when switching from a digital channel to another (analogor digital) channel is first switching off the video and audio decoders204, 208 as shown by label 1001, thus having no video (black) on thescreen and silence on the output (speakers).

For the switch to digital case, only after the decoders 204, 208 havebeen switched off, the control software 1003 instructs thetuner/demodulator to retune to the new desired frequency and achieveQAM-lock, as illustrated by label 1002. After the modulator 201 hasachieved QAM lock, the demux 202 starts receiving a valid signal, andwill be instructed by the control SW to filter required data, video, andaudio packets. Subsequently, the control SW will switch on the video andaudio decoders 204, 208 which will start producing audio/video (afterthe buffers 203, 207 are sufficiently full).

For the switch to analog case, the situation is similar, as shown inFIG. 11. Only after the decoders 204, 208 have been switched off, thecontrol software 1003 instructs the tuner 200 to retune to the newdesired frequency. After tuning is complete, the analog video and audioprocessors 301, 302 start receiving a valid signal, and will immediatelystart producing video and audio. As soon as the tune is complete, thecontrol software will instruct the switches 304 and 303 in the backendto switch the output signals to the output of the analog audio/videoprocessors (via control 1003 as depicted in the FIG.).

The common channel change procedure just described is disadvantageoussince the output is set to black/silence while there still is validvideo and audio data in the decoder input buffers (whether the switch isfrom digital to digital or from digital to analog). An illustrativeembodiment of the present invention exploits the presence of this databy not resetting the decoders 204 and/or 208 before starting the tune.Essentially, all steps as just described remain the same, except theaudio and video decoders will not be switched off (i.e., step 1001 isomitted). This means that the audio/video decoders will keep playing,(potentially as much as half a second, or even more) until their inputbuffers are empty. At that point the output signals would go toblack/silence due to absence of data.

Method 3 (General): Avoiding Video ‘Jump’ in A/D or D/A Switch

Usually, when switching from a digital channel to an analog one theother way around, the video might ‘jump’, caused by losingsynchronization, which is disadvantageous to the viewer's experience.This jump is even visible when the video itself is black. In analogvideo signals, the video sync is taken from the input signal itself. Inthe digital case, the receiver has to generate the synch itself. Thesetwo different synch signals are most likely not exactly aligned, meaningthat the end of one frame at one signal is when the other synch signalis in the middle of refreshing a frame, leading to the ‘jump’ in thescreen.

One option is to force the receiver to use its own generated synchsignal for the analog video as well. Another option is to only turn onthe video backend (renderers) when the synch is in the invisible region(so-called VBI—Vertical Blanking Interval).

Method 4 (General): Audio Ramp Down/Up

Similarly, when tuning to another signal, audio might cause some audibleartifacts due to the sudden switch-off of the signal. This can be maskedby a ramp-down just before the desired switch, and a ramp-up just afterthe switch is complete. The ramp-down/up periods could be as short as1/10^(th) of a second as indicated in FIG. 7, where the actual start oftune is indicated by 701.

System for Personalized Messaging

In situations such as personalized advertising/messaging, one (digital)channel can carry one or more messages, one of which can be used tooverlay the message on another (main) analog or digital channel. Theexact message to play will be decided by the receiving equipment. Thissystem is disclosed in FIG. 8. The system depicted typically is part ofa broadcast headend. This headend receives a network feed 801 (analog ordigital) coming from the content provider. This feed contains televisionprogramming 800. The programming also contains messages 803, such ascommercial. Furthermore, the programming contains so-called cue-tones802 that indicate the start of a message 803.

To support message replacement by the receiver at the consumer home, theheadend contains a cue-tone detector 804 that detects the cue-tones 802in the source programming. The cue-tone detector passes the incomingstream on virtually unchanged. The only change is that it takes out thecue-tone 802 and optionally replaces it by another trigger message 810that can be used by the receiver in the consumer's homes to detect theswitch moment. Cue-tone 810 might even be identical to 802, in whichcase nothing is changed in the stream.

In case channel 800 is analog, message 810 is typically encoded as VBIdata, audio tones, or other analog encoded data. Another possibility isto use time-based switching, in which case the receivers have a clockand the network will generate time-stamped switching instructions thatare sent ahead of time to the receivers. The receivers will receivethese instructions, and execute the switch on the exact time specifiedin the instruction. This approach docs require clock synchronizationbetween central clocks and receiver clocks. There are several (wellknown) methods of doing this, such as: “slaving” all receiver clocks toa central master clock (as is done in some Access Control systems),constantly measuring the “drift” in the various receiver clocks andadjusting the system for it, etc,.

In case channel 800 is digital, message 810 can be encoded in manydifferent ways. In MPEG-2 systems the message can be encoded as privatedata, an MPEG splice-point message, etc. If the receiver has nocapability of decoding any of these messages, there is again the optionof using time-based return switching in which the receivers receivetime-stamped return-switching instructions.

In personalized messaging it is advantageous to place alternativemessages on a separate channel as also depicted in FIG. 8. This way, thealternative channel can be shared for overlaying messages over manydifferent channels. In FIG. 8 this is accomplished by the cue-tonedetector providing playout triggers 805 to a digital server 806 whichhas the alternative messages stored on disk 807. When receiving aplayout trigger, the digital server takes the appropriate message fromits storage and plays it out on the separate channel 809.

The cue-tone detector and digital server are synchronized such that thealternative message 811 falls within the time window of message 803 inthe main programming. Note that message 811 might be shorter thanmessage 803, to allow for switching by the receiver from the mainprogram to the alternative message, and back (without loosing content).Each switch might be up to 2 sec. as explained before. Also, some extratime might be added to allow for streams 808 and 809 to be drifting,e.g., because they are generated using a slightly different clock. Intotal, the amount of time the alternative message is shorter than themessage that is overlaid in the main program is determined by the worstcase situation, that has to be measured in the field with the deployedreceivers.

Digital server 806 will insert a separate trigger message 812 in thechannel that is used by the receiver to switch back to the mainprogramming channel after the alternative message is finished. Message812 can be encoded in many different ways since channel 809 is digital.For example, in MPEG it can be encoded as private data or any other typeof data that can be detected by the receiver.

All television channels, including the ones just discussed, aresubsequently combined and positioned as the right frequency in thefrequency spectrum by combiners/upconverters 815. The resulting RFsignal 816 is leaving the headend and passed on downstream, eventuallyreaching the receivers at the consumer's homes. Note that digitalchannels first pass through a modulator 814 that converts the digitalbitstream into an analog signal.

Note that the system that has just been disclosed can be used in anysituation where parts of a television program can be replaced by otherparts that are located on one or more different channels.

Next, methods will be disclosed that can reduce the switching time by areceiver between streams 808 and 809 by preparing streams 809 ahead oftime in the system as just described.

Method 5 (Specific): Sending Control Messages Ahead of Time

The first method consists of providing the basic control messages (suchas PMT, PAT) for the new stream ahead of time (i.e., before the switch).This can be done in a number of ways.

One option is to standardize the content of the control messages in thereceiver software (for instance in an MPEG-2 environment: always usefixed PIDs (packet Identifiers) for the destination digital stream inpersonalized messaging.

Another option is to send the content of the control messages to thereceiver as part of the analog program, before the switch takes place(e.g. insert the control messages in the VBI data of the analogprogram).

Yet another option is to send the content of the control messages to thereceiver via another link (e.g., in an MPEG-2 cable system via anout-of-band channel, in DTH system via phone modem).

This approach will ensure that the receiver does not have to wait forthe messages to appear in the stream that has been switched to, sayingup to 50 msec, of time.

Method 6: (Specific) Reduce Time Needed to Find Next I-Frame

The second method is ensuring that the video in the alternative messagecan be decoded right from the beginning. In an MPEG environment thismeans that the alternative messages starts with an I-frame. This savesthe decoder from waiting for the next I-frame before it can begin itsdecoding process. This technique can save as much as 500 msec, of time.Since the alternative message can be prepared ahead of time, andinsertion of the message is controlled b the trigger messages in themain program, this is readily achieved.

Method 7: (Specific) Reduce Required Buffer Fullness

The third method is ensuring that video presentation starts as soon aspossible (low latency). This can be achieved by reducing the initialVBV-delay for the alternative message. This can be done by either(temporarily) increasing the bitrate of the video (so that the buffer isfilled quicker), or by encoding the video such that it needs less datain the buffer before encoding (e.g., by forcing a lower-maximum VBVbuffer size for the encoder). This invention can save as much as 200msec. This would mean in FIG. 4 that the initial ramp-up will take ashorter amount of time, either because the ramp-up is steeper (higherbitrate) or by the encoder encoding the video such that the buffer isless full in the beginning.

Together, methods 5-7 can lead to a saving of up to 750 msec, which isconsiderable. Method 2, in addition, can lead to a saving of up 300msec. Thus, the total time needed for the switch can be reduced withmore than a full second by employing methods 2,5,6,7 together. This canbe achieved without any hardware modifications of existing receivers.

Method 8: (Specific) Switch Back to a Scramble Program

To descramble a digital stream certain control messages must be receivedbefore actual descrambling (and, consequently, video/audio presentation)can start. Examples of such messages are control words such as ECMs andEMMs (Entitlement Control Messages and Entitlement Management Messages).

Descrambling has an impact on Digital-to-Digital Switching, specificallyif the original digital channel is scrambled. If a receiver switches toa scrambled stream, in addition to the steps discussed earlier, it willneed some time to receive the Broadcast Access Control messages in thestream (which it needs for descrambling). This causes additionalswitching delay. An example of this occurs when the receiver is playinga scrambled stream, switches over to another stream to play out apersonalized message (unscrambled), and then switches back to theoriginal stream (at which point the additional delay will occur).

There are several ways of solving this issue. The preferred solutionwould be to ensure that the receiver, before switching from a stream Ato a scrambled stream B, already has access so the Broadcast AccessControl messages for stream B. This can be done by sending thesemessages via an electronic link (like a modem) to the receiver andstoring them there until needed. Another approach is to simply embed(copy) the Broadcast Access Control messages from stream B in theoriginal stream A. This is illustrated in FIG. 9. This will ensure thatthe receiver will incur no additional delay in waiting for the BroadcastAccess Control messages, and therefore no additional switching delay isvisible to the viewer(s).

In a situation where it is desired that the alternative messages arealso scrambled, it is a good option to scramble them using the samecontrol words as used in the main program. This way, the receiver canswitch back and forth between the channels without delays (since thesame scrambling is used).

Each of the described methods provides some optimization of interstreamswitching. The present invention includes any possible permutation orcombination of these methods. An illustrative embodiment of the presentinvention for optimizing switching from an analog to a digital channelincludes the combination of methods 3, 4, 5, 6 and 7. An illustrativeembodiment of the present invention for optimizing switching from adigital channel to an analog channel includes the combination of methods2, 3 and 4. An illustrative embodiment of the present invention foroptimizing switching from a digital to another digital channel includesthe combination of methods 2, 3, 4, 5, 6 and 7. In case that the digitalchannel is scrambled, then method 8 may be included to optimize theswitching.

Although the invention has been shown and described with respect toillustrative embodiments thereof, various other changes, omissions andadditions in the form and detail thereof may be made therein withoutdeparting from the spirit and scope of the invention. It will understoodthat various modifications may be made to the embodiments disclosedherein. Therefore, the above description should not be construed aslimiting, but merely as exemplification of the various embodiments.Those skilled in the art will envision other modifications within thescope and spirit of the claims appended hereto.

What is claimed is:
 1. In a broadcast television receiver including atuner for selecting one channel from multiple channels and an encodedmedia buffer for receiving digital encoded media from a channel selectedby said tuner, a method of switching from a first digital channel to asecond channel comprising: inputting digital encoded media from saidfirst digital channel selected by said tuner to said encoded mediabuffer; halting input of digital encoded media from said first digitalchannel to said encoded media buffer, while continuing to output digitalencoded media from said first digital channel from said encoded mediabuffer; switching said tuner to said second channel; and after a passageof time for said tuner to complete switching to said second channel,outputting said second channel from said tuner.
 2. The method of claim 1wherein said second channel is a digital channel, and said step ofoutputting said second channel from said tuner includes commencing inputof digital encoded media from said second channel to said encoded mediabuffer.
 3. The method of claim 1 wherein said second channel is ananalog channel, and said step of outputting said second channel fromsaid tuner includes bypassing said encoded media buffer.
 4. The methodof claim 1 wherein said passage of time for said tuner to completeswitching to said second channel is a predetermined amount of time, anda quantity of digital encoded media in said encoded media buffer ismaximized to cover a maximal amount of said predetermined amount oftime.
 5. The method of claim 1 further including: before said step ofswitching said tuner to said second channel, decreasing audio volume forsaid first channel; and after said step of switching said tuner to saidsecond channel, increasing audio volume for said second channel.
 6. Themethod of claim 2 wherein said media is digital video.
 7. The method ofclaim 6 wherein said second digital channel is an MPEG encoded channel,and said second digital channel is created such that after said step ofswitching said tuner to said second channel, upon commencing input ofdigital encoded video from said second digital channel to said encodedvideo buffer, the first input into said encoded video buffer is an MPEGI-frame.
 8. The method of claim 6 wherein said step of commencing inputof digital encoded video for said second digital channel to said encodedvideo buffer includes previously encoding said digital encoded videosuch that a VBV-delay of a first video frame in presentation order isreduced.
 9. The method of claim 8 wherein said VBV-delay of a firstvideo frame in presentation order is reduced by increasing the videobitrate.
 10. The method of claim 8 wherein said VBV-delay of a firstvideo frame in presentation order is reduced by reducing a VBV buffersize maximum value for an encoder.
 11. The method of claim 1 furtherincluding sending at least one control message for said second channelto said broadcast television receiver at a time before said step ofswitching said tuner to said second channel.
 12. The method of claim 1wherein said media is video, MPEG encoded video, audio, MPEG encodedaudio, or AC-3 encoded audio.
 13. A broadcast television receiver,comprising: a tuner, to select one channel from multiple channelsreceived by said receiver; a digital encoded media buffer coupled tosaid tuner; said digital encoded media buffer to receive digital encodedmedia when a first digital channel is selected by said tuner, whereinwhen said tuner is switching to a second channel, said digital encodedmedia buffer ceases to receive digital encoded media for said firstdigital channel, but continues to output digital encoded media for saidfirst digital channel through an output; and wherein after said tunerhas completed switching to said second channel; if said second channelis a digital channel then said digital encoded buffer receives digitalencoded media for said second channel.
 14. The broadcast televisionreceiver of claim 13 wherein said second channel is an analog channel,and output from said tuner bypasses said digital encoded media buffer.15. The broadcast television receiver of claim 13 wherein a quantity ofdigital encoded media in said encoded media buffer is maximized to covera maximal amount of switching time required by said tuner.
 16. Thebroadcast television receiver of claim 13 wherein audio output from saidreceiver is decreased in volume while said tuner is switching to saidsecond channel.
 17. The broadcast television receiver of claim 13wherein said media is digital video.
 18. The broadcast televisionreceiver of claim 17 wherein said second digital channel is an MPEGencoded channel, and said second digital channel is created such thatwhen said digital encoded buffer receives digital encoded media for saidsecond channel, the first input into said digital encoded buffer is anMPEG I-frame.
 19. The broadcast television receiver of claim 17 whereinsaid digital encoded video for said second channel is previously encodedsuch that a VBV-delay of a first video frame in presentation order insaid digital encoded buffer is reduced.
 20. The broadcast televisionreceiver of claim 19 wherein said VBV-delay of a first video frame inpresentation order is reduced by increasing the video bitrate.
 21. Thebroadcast television receiver of claim 19 wherein said VBV-delay of afirst video frame in presentation order is reduced by reducing a VBVbuffer size maximum value for an encoder.
 22. The broadcast televisionreceiver of claim 13 wherein at least one control message for saidsecond channel is sent to said broadcast television receiver at a timebefore said tuner has completed switching to said second channel. 23.The broadcast television receiver of claim 13 wherein said media isvideo, MPEG encoded video, audio, MPEG encoded audio, or AC-3 encodedaudio.